Amp wiring question

I have a stock ls400 90 model and i was wondering if it was possible to run the signal wire that goes from the HU to the sub amp into an aftermarket amp, basically to give a better bass sound ( and replace the sub ). Ive located the amp on the left side of the boot but cant seem to determine which wire is for the signal. I'm not talking about the two wires that go to the sub i'm after the signal wire from HU to amp. I was hoping that there would be an rca wire but no such luck. Any help would be welcome, thankx

I have a stock ls400 90 model and i was wondering if it was possible to run the signal wire that goes from the HU to the sub amp into an aftermarket amp, basically to give a better bass sound ( and replace the sub ). Ive located the amp on the left side of the boot but cant seem to determine which wire is for the signal. I'm not talking about the two wires that go to the sub i'm after the signal wire from HU to amp. I was hoping that there would be an rca wire but no such luck. Any help would be welcome, thankx